Wednesday, July 18, 2012

The first gene transfer (HGT) network (1910)

I have previously noted in this blog that the first two published phylogenetic networks (by Buffon in 1755 and Duchesne in 1766) were hybridization networks. This leads to the obvious question: what was the first phylogenetic network illustrating horizontal gene transfer (HGT)?

This depends, of course, on exactly how one defines "HGT". If we require explicit reference to genes, then this must post-date the origin of our current understanding of genetics and the nature of genetic material. The first description of HGT is usually credited to Victor Freeman (1951), which thus sets an earliest possible date. However, it will take quite some bibliographic investigation to work out who first illustrated this with a phylogenetic network (none of the earliest reports were concerned with phylogeny). [See the later post The first HGT network.]

However, if we consider HGT to be a subset of genome transfer (or genome fusion), which is the horizontal transfer of an entire organismal genome, then a much earlier date becomes possible. This is because the idea of endosymbiosis, which posits eukaryote organelles as the acquisition of different bacterial genomes, dates back more than a century.

For example, Constantin Mereschkowsky (1905) developed his symbiogenesis theory with the explicit goal of explaining the evolutionary development of land plants from algae-like forms of life, postulating that chloroplasts originated as symbiotic blue-green algae. Richard Altman (1890) had already proposed (indirectly) that what we now call mitochondria are also symbionts.

Mereschkowsky (1910) then took this idea further, and developed a scenario for the origin of the nucleus and cytoplasm from two kinds of organisms and two kinds of protoplasm, called mycoplasm and amoeboplasm. Each kind of protoplasm had an origin in different historical epochs. He illustrated this two-stage symbiosis idea with an explicit network, which appears on page 366 of his paper.

Click to enlarge.

Mereschkowsky's own interpretation of this diagram as a genome-transfer network thus seems clear enough, even though he makes no explicit reference to a genome.